1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
In this specification, a semiconductor device generally means any device which can function by utilizing semiconductor characteristics, and an electro-optic device, a semiconductor circuit, and an electronic appliance are all included in the category of the semiconductor device.
2. Description of the Related Art
A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to the transistor; in addition, an oxide semiconductor has attracted attention as another material.
For example, a transistor whose active layer includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of less than 1018/cm3 is disclosed (see Patent Document 1).